1. Field of the Invention
The present invention relates to a frame unit for tensioning a printing screen, often alternatively referred to as a printing stencil, for use in a screen printing machine, and a jig for fitting a printing screen to or removing a printing screen from a frame unit.
A printing screen, typically comprising a screen sheet formed of a metal or plastic, includes apertures which define the pattern to be printed, with the screen being located over an object to be printed and printing medium forced through the apertures to provide the required print.
2. Description of Related Art
In order to produce high-quality prints, printing screens have to be held under tension. Numerous frame units have been developed which allow for the tensioning of printing screens, with examples being disclosed in U.S. Pat. No. 5,606,911, U.S. Pat. No. 5,606,912, U.S. Pat. No. 5,941,171, U.S. Pat. No. 6,038,969, U.S. Pat. No. 6,067,903 and U.S. Pat. No. 6,289,804.
Whilst these existing frame units all provide for the tensioning of printing screens, the frame units suffer from a number of drawbacks, in particular in requiring the printing screens to include stress-relieving features to enable the uniform tensioning of the printing screens, and in being of relatively complicated construction and thus relatively high cost. The application of a uniform tension over a printing screen is particularly important in order to obtain a high-quality print, as an uneven tension can lead to rippling of the screen sheet of the printing screen, which rippling results in poor-quality printing. The existing frame units utilize continuous rigid members to apply tensioning forces to the edges of a printing screen, but these members, in being rigid, and as a consequence of manufacturing tolerances, apply slightly different tensions at the points of attachment along the edges of the printing screens.